1. Technical Field
The present invention discloses a method and structure for electrically joining two substrates.
2. Related Art
FIG. 1 depicts a top view of a substrate 10 with a two-dimensional array of electrically conductive pads 12 (e.g., gold or gold-plated pads) on a surface of the substrate 10, in accordance with the related art. The substrate 10 is an electrical substrate such as, inter alia, a printed wiring board or an electronic module (e.g., a module of a chip carrier with one or more attached semiconductor chips).
FIG. 2 depicts a cross-sectional view of an electrical structure 13 comprising substrates 14 and 18, each such substrate being of the type shown in FIG. 1. As an example, the substrate 18 may include a printed wiring board and the substrate 14 may include an electronic module. The substrate 14 has electrically conductive pads 16, and the substrate 18 has electrically conductive pads 20. A conductive coupler 22 permanently electrically couples the substrate 14 to the substrate 18. The conductive coupler 22 may be, inter alia, a solder ball, a solder column, etc.
A problem with the related art of FIG. 2 is that electrical structure 13 is vulnerable to solder fatigue and failure at a contact surface 17 between the conductive pad 16 and the conductive coupler 22, or at a contact surface 21 between the conductive pad 20 and the conductive coupler 22. For example, the failure could result from thermal strain on the conductive coupler 22 introduced during temperature transients, said thermal strain resulting from differential coefficient of thermal expansion (CTE) between the substrate 14 and the conductive coupler 22, between the substrate 18 and the conductive coupler 22, between the substrate 14 and the substrate 18, etc. Accordingly, there is a need for a method and structure that reduces the probability of such failure.
Another problem with the related art of FIG. 2 is that the electrical structure 13 cannot be easily repaired or upgraded in the field. Accordingly, there is a need for a method and structure that facilitates repairing or upgrading the electrical structure 13 in the field.